The present invention relates to a pressure sensor, particularly to a pressure sensor having plural pressure sensitive diaphrams and providing electric signals indicating at least two pressures as its outputs.
With the increase of social requests such as economy of fuel and/or purification of exhaust gas, various controls of an engine, for example controls of fuel injection, ignition timing and recirculation of exhaust gas, come to be electronically accomplished by utilizing a micro-computer. In such electronic controls, various kinds of pressure signals, for example signals respectively indicating absolute pressures of atmosphere and vacuum in a manifold of the engine and signal indicating relative pressure of the vacuum in the manifold to the atmospheric pressure, are necessary to be detected accurately, and various kinds of pressure sensors have been proposed.
Such pressure sensor for use in an automobile, however, must satisfy various requirements, as well as of the accuracy, of reliability and cost. Namely, a pressure sensor having high reliability, structure suitable for mass production and low cost is required.
For example, in Japanese Patent Publication No. 52-17437 (1977), a pressure transducer including a piece of single-crystal chip of semiconductor such as silicon, on which two pressure sensitive diaphragms are fabricated, is shown. In this pressure transducer, pressure to be detected is applied to one of these pressure sensitive diaphragms, and atmospheric pressure to the other thereof. On the backs of the pressure sensitive diaphragms, strain gauges are constructed, and a vacuum chamber or a chamber in which inactive gas is filled is provided thereon. And this pressure transducer provides only with an electric signal in proportion to the relative pressure of the gas to be detected to atmosphere.
Such pressure transducer, however, has following drawbacks. That is, because the semiconductor single-crystal chip is attached by bonding pads on a housing member which is composed of material such as iron having a coefficient of thermal expansion being greatly different from that of single silicon crystal, errors appear in the output of the pressure transducer. In the case that a chip of the single silicon crystal with the pressure sensitive diaphragm(s) is bonded on the housing of metal, as is well known in the art, strain appears in the pressure sensitive diaphragm depending on the change of ambient temperature because of the difference between the coefficients of the thermal expansions thereof, and therefore causes the errors.
Also, the variety of the two pressure sensitive diaphragms in their characteristics, contributes to the errors. That is, when the relative pressure is obtained by detecting two pressures different to each other, though it may be the same when the two pressures are respectively obtained, the uniformity of the characteristics of the pressure sensitive diaphragms are required in particular. If the semiconductor single-crystal chip is fixed on the metal housing by the bonding pads, it causes great difficulty to obtain two pressure sensitive diaphragms being very similar to each other in characteristics. Namely, it is very difficult to accomplish the fixation under the uniform bonding condition on the whole surface between the chip and the housing.
Further, the pressure transducer mentioned above is not suitable for the mass production in view of the structure thereof. Particularly, it is true in the structure of the electrodes which are provided for electrically connecting the strain gauges within the vacuum chamber to the outside.
Furthermore, the pressure transducer provides only with a signal indicating one pressure, i.e. the relative pressure of the gas to be detected with respect to atmosphere.
Other relevant prior art is as follows:
(1) U.S. Pat. Nos. 3,918,019 and 4,079,508, "MINIATURE ABSOLUTE PRESSURE TRANSDUCER ASSEMBLY AND METHOD"s. PA0 (2) U.S. Pat. No. 3,397,278, "ANODIC BONDING".
Although both of these patents teach a glass substrate and Anodic Bonding technique, there is no discription or teach concerning a semiconductor single-crystal chip having two pressure sensitive diaphrams.
This patent relates to the Anodic Bonding technique.